Fatigue-limited metal components of gas turbines or jet engines, or other machine components subject to metal failure or fatigue, must be carefully managed in order to avoid failure during operation. The failure, for example, of a critical component of a jet engine during operation may result in loss of life or other catastrophic consequences. Currently in the aviation industry (commercial and military), there are three general types of management techniques or approaches used for the management of fatigue-limited machine components in order to prevent possible catastrophic failure due to metal fatigue. Each of these approaches attempts to balance safety and economic concerns based on available data. See, for example, S. Suresh, Fatigue of Metals, 499-502 (1991), which generally discusses the three commonly used management approaches.
The most conservative of these approaches, often termed the "safe life" approach, is based on the estimated fatigue life established through analysis and comparable experience by the engine manufacturer. This approach attempts to estimate the point at which the shortest-lived part or component in the total population would be expected to fail. After allowing for a suitable safety margin, an arbitrary retirement point is adopted for that component. This retirement point is normally measured in total take-off cycles or hours. Once a part reaches the retirement point, it is removed from service and mutilated to prevent further, unauthorized use. Although generally allowing for the greatest margin of safety, significant economically useful service life of such parts is lost. In effect, this "safe life" approach is based on, and controlled by, an estimation of the lifetime of the weakest part or component in the total population.
A somewhat less conservative management technique is the so-called "fail safe" approach. In this approach, a maximum service life is determined by the total accumulated service hours or cycles (whichever is shorter) at which the first crack is detected in an actual part (i.e., disk or drum rotor used in a compressor, turbine, or engine) in the population of like parts. Once a part has developed such a crack, its accumulated service life (in hours or cycles) is effectively used to determine the service life of all similar parts in the population. If a part is later found to develop a crack at an earlier time, then that part is then used to redefine (and shorten) the acceptable service life limits of the population. Once a part reaches its acceptable service life, it is removed from service and mutilated to prevent further use. In effect, this "fail safe" approach is also based on, and controlled by, the actual weakest part or component in the total population. Many parts may still have many hours of safe and useful service life remaining beyond that of this weakest part. But, since the useful and safe service life of these parts cannot be reliably determined, they must be removed from service in the interest of safety. This "fail safe" approach is generally used in the airline industry for mature fleets where low cycle fatigue cracks have been detected in the relevant component populations. Where sufficient service data has not been developed, the more conservative "safe life" approach is generally used. In each approach, however, parts having many remaining hours of safe and reliable use will be removed from service.
More recently, the United States Air Force has successfully adopted an even less conservative management technique, the so-called "retirement for cause" approach, for its management of some critical engine components. In this approach, the parts are periodically examined non-destructively for cracks using, for example, fluorescent dye penetration or magnaflux techniques. Once a crack is observed, that part, but only that part, is immediately retired from service. Other parts, even though they may have accumulated service times equal to or greater than the retired part, are continued to be used until they actually develop cracks. To operate safely, this approach requires periodic and frequent inspections of the individual parts. In general, as parts age, the frequency of inspections should be increased. In any event, the frequency of inspections must be such that the period between inspections is less, preferably by a significant margin, than the time normally required for a detectable crack to further deteriorate to the point of actual failure. Although this approach may result in more frequent teardowns for inspection of the individual parts, the potential savings based on achieving, or at least approaching, the maximum lifetimes of the individual parts can be enormous. The major drawback of this approach is that it relies upon detection of an actual crack in the part. Thus, this approach is generally not suitable for parts in which crack formation cannot be detected in a reliable and consistent manner. Once a crack has formed, the part contains, in effect, a permanent, irreversible defect which will ultimately lead to failure, perhaps catastrophic failure, unless that part is removed from service in a timely manner. Additionally, this approach, of course, is not suitable for use where the normal time between the initial development of a detectable crack and failure of the part is relatively short. Moreover, in parts where actual failure normally does not follow quickly after the development of a crack, if such a crack develops shortly after an inspection, the risk of failure during actual operation increases simply because the length of time in which the part is operated with the defect is maximized. Therefore, this method has an increased safety risk when compared to the "safe life" and "fail safe" approaches. This increased risk, although perhaps small, may still be significant because the detection point is the actual formation of a detectable crack. The longer that part remains in service, once a crack has formed, the greater the risk of catastrophic failure.
It would be desirable, therefore, to provide non-destructive methods to measure the remaining service or useful life of fatigue-limited metal components before crack initiation has begun or, at least, before actual cracks can be observed (i.e., before permanent and irreversible damage has begun). It would also be desirable to provide methods by which the service or useful life of fatigue-limited metal components could be increased without significantly increasing the risk of catastrophic failure of the metal components during operation. Such methods would provide both increased safety and economy for the aviation industry (commercial and military). The methods of this invention generally provide such improved methods.